The present invention relates to a thermal printing system (or thermal printer) using a thermal head for changing, according to input printing data, a color of proper parts of a recording sheet by causing the thermal head to come in contact with the recording sheet appropriately so as to print an image corresponding to the input printing data on the recording sheet. Such a recording sheet comprises a paper which is normally specially coated so that the special coating discolors when it is heated. One example of such a thermal printing system comprises a facsimile apparatus. The present invention in particular relates to a thermal printing system having a function for preventing the thermal head from being overly heated and having a function for preventing partial loss of printed information. Such partial loss of printed information may occur because the above-mentioned thermal-head over heating prevention function prevents the thermal head from further heated due to excessive rising of the temperature of the thermal head being detected.
In one example of such a facsimile apparatus according to related art, temperature detecting means such as a thermistor is provided for detecting a temperature of the thermal head. If the detected temperature of the thermal head reaches a predetermined value, operation of a printing strobe is terminated so as to stop heating of the thermal head. As a result of the heating stoppage, overly heating of the thermal head is prevented.
However, termination of the operation of the printing strobe such as mentioned above may result in a partial loss of printed information if the printing strobe operation termination occurs in the middle of the printing work. On the other hand, it may be possible, in order to prevent the termination of the operation of the printing strobe, to provide measures for reducing heat load applied to the thermal head while the printing strobe is operated. Such measures may comprise, for example, changing the resistance value and/or electric current associated with the thermal head, and/or adding a heat radiation plate into the thermal head. These measures are used for preventing heat from accumulating in the thermal head and thus for preventing the thermal head from being overheated. In this method, partial loss of printed information can be prevented but the thus printed information may become degraded due to insufficient heating of the recording (or printing) paper by means of the thermal head. Such insufficient heating may result from the above-mentioned reduction of the energy supplied to the thermal head.
Further, in the facsimile apparatus according to the related art, a printing strobe-width or strobe time-width (hereinafter used to refer to, a time period for which energy is applied to the thermal head) is determined in response to the magnitude of the detected temperature of the thermal head. Thus, a uniformity in the printing density can be realized even though the temperature of the thermal head tends to vary due to certain reasons, such as variation in ambient temperature and variation of current and/or voltage of the power source used for heating the thermal head.
In this method, however, a problem may occur in a case where a portable power source is utilized for heating the thermal head in the facsimile apparatus. The problem may occur due to a variation in the current and voltage of the portable power source, which variation occurs significantly in particular in a power source such as a portable power source. It is difficult to maintain a proper printing density only by altering the printing strobe time-width to correct the variation in the thermal head temperature due to the existence of such a significant variation in the current and voltage of the power source for heating the thermal head.